1. Field of the Invention
The present invention relates to a fixing device favorably for use in an electrophotographic image forming apparatus, and to an image forming apparatus having the same.
2. Description of the Related Art
In an image formation using an electrophotographic system, a photoreceptor charged with a uniform electric potential is exposed to a light in accordance with image information so that an electrostatic latent image is formed. The formed electrostatic latent image is developed by a developer so as to be visualized. The visualized image is transferred on a recording paper or the like, and the transferred developer on the recording paper is made to be fixed so as to form a solid recording image.
The fixing device used for such image formation, is generally composed of a heating roller and a pressure roller, which are such configured that unfixed developer is fused and fixed by heating of the heating roller and pressing of the pressure roller when passing the recording paper on which the developer for forming a visualized image through a contact region (hereinafter sometimes referred to as a nip section) of the both rollers, which region is formed by pressing the pressure roller against the heating roller.
During a fixing operation in the fixing device, there sometimes occurs a so-called hot offset that the developer fused on the nip section of the both rollers is not all fixed on the recording paper, but a part of the developer is attached to a surface of the roller. For instance, the developer attached to the heating roller is transferred on a portion which should be properly a white base, on a recording paper on which the developer is to be subsequently fixed, with the result that an image defect is made to occur.
Moreover, on the pressure roller, the developer which has already fixed to a back surface of the conveyed recording paper, for instance as in a case of duplex print, may be sometimes fused again by heat when passing through the nip section and a part of the developer may be transferred and attached to the pressure roller. The developer thus attached to the pressure roller may cause the image defect and further, may cause a soil of the back surface of the recording paper.
The image defect caused by the hot offset in the fixing device sometimes remains, in a case of black-and-white print, mere defects such as a fog in a white base of the formed image, a soil on the back surface of the recording paper, or the like in a tolerable range. However, in a case of full-color print, since a developer having a color different from a prescribed one is transferred from the both rollers, there often occur practically intolerable defects.
As a related art for solving such a problem, there is an apparatus having roller cleaning means on the both rollers provided in a fixing device (refer to Japanese Unexamined Patent Publication JP-A 2003-107952).
FIG. 11 is a schematic view showing a configuration of roller cleaning means 1 provided in a related art fixing device. FIG. 11 illustrates the roller cleaning means 1 provided on a heating roller 2 in the fixing device.
The roller cleaning means 1 comprises a feeding roller 4 for feeding a belt-shaped cleaning member 3 which has been previously rolled up, a winding roller 5 for taking up the cleaning member 3 fed from the feeding roller 4, and a pressure-contact roller 6 (also referred to as a web pressure-contact roller) provided between the feeding roller 4 and the winding roller 5 so as to press the cleaning member 3 against the heating roller 2.
The roller cleaning means 1 presses the cleaning member 3 against the heating roller 2 which rotates in an arrow sign 7 direction in a state where the winding roller 5, the feeding roller 4, and the pressure-contact roller 6 are made to be at rest without being made to rotate so that the heating roller 2 and the cleaning member 3 are made to be slidingly scrubbed. By so doing, a developer 8a attached in a fused state to an outer circumferential surface of the heating roller 2 is removed, and the removed developer 8b is accumulated, still in a substantially fused state, in a gap formed by the cleaning member 3 located between the pressure-contact roller 6 and the winding roller 5, and the surface of the heating roller 2.
A cleaning capability is lowered when the developer 8b accumulated in the gap is excessively accumulated. Accordingly, when the developer 8b accumulated in the gap reaches a certain amount level, the roller cleaning means 1 feeds the cleaning member 3 by making the feeding roller 4 rotate in an arrow sign 10 direction, as well as takes up the cleaning member 3 by making the pressure contact-roller 6 rotate in an arrow sign 11 direction and further operates the winding roller 5 for take-up in an arrow sign 9 direction, with the result that the developer 8b is made to be detached from the surface of the heating roller 2 in a state where the developer 8b is attached to the cleaning member 3.
When this feed amount of the cleaning member 3 due to the feeding roller 4 is small, a portion of the cleaning member 3 which has been used once for cleaning cannot be completely renewed. Consequently, it is impossible to sufficiently recover the cleaning capability in spite of a fact that the cleaning member 3 is fed out. In contrast, when the feed amount of the cleaning member 3 is too large, a clean portion of the cleaning member 3 is unnecessarily consumed. Consequently, a running cost is increased.
In addition, when a take-up amount due to the winding roller 5 is small with respect to the feed amount of the cleaning member 3 due to the feeding roller 4, the fed cleaning member 3 is made impossible to show a sufficient cleaning capability by being slackened between the feeding roller 4 and the winding roller 5. In contrast, when the take-up amount due to the winding roller 5 is large with respect to the feed amount of the cleaning member 3 due to the feeding roller 4, an excessive tension is loaded on the cleaning member 3. In a case where the tension is extreme, the cleaning member 3 may be broken apart.
As described above, a control of the feed amount and take-up amount of the cleaning member 3 is extremely important in the cleaning means 1. However, there is neither disclosure nor suggestion on this matter in Japanese Unexamined Patent Publication JP-A 2003-107952.